Spectrophotometers referred to as Fourier transform spectrophotometers are well known. Each of these instruments is essentially composed of an interferometer that produces an interferogram from a light beam directed into the entry of the instrument. Measurements of light intensity are recorded, which respectively correspond to fixed values of an optical path length difference. This optical path length difference exhibits identical spacings between two successive measurements in the interferogram. The spectral distribution of the light in the beam is then evaluated by calculating a Fourier transform of the measured intensities as a function of the values of the optical path length difference.
Now, the reduction of the interferogram to a limited sample of light intensity measurements leads to an uncertainty in the frequency to which a spectral component intensity, calculated by Fourier transform, corresponds. This problem is well known and is referred to as ‘aliasing’. In order to eliminate this uncertainty, the interferometer is combined with a filter within the spectrophotometer, which filter selects a part of the light from the light beam. In this manner, the light producing the interferogram corresponds to a spectral interval which is determined by the filter. The frequencies to which the calculated spectral component intensities correspond then necessarily belong to this interval.
The filter has to be chosen as a function of the spectral interval of analysis: it must have a filtering window whose limits correspond to those of the analysis interval. But, a filter that corresponds to any given spectral interval is not always available. The limited variety of the available filters therefore leads to a constraint on the spectral interval within which a light beam may be analysed using a Fourier transform spectrophotometer. Such a constraint can be a problem, especially for measurements in space performed from a satellite, since the spectrophotometer filter cannot be easily changed.
An object of the present invention consists in eliminating the necessity for using a filter in order to perform spectrophotometric measurements by sampling of an interferogram.